In a mobile radio system, the emitted signal can follow different paths to reach the receiver, due to reflections by obstacles. This phenomenon creates a multipath channel and interference among a large number of multipaths with different time delays. The received signal can therefore be selectively faded for some frequencies.
Within a Code Division Multiple Access (CDMA) receiver, the selection of the relevant paths/delays of the transmission channel is needed in order to perform the channel estimation and data detection in presence of noise and interference the RAKE receiver has to combine coherently the different estimated paths of the channel. Due to noise, interference and Doppler spread, some paths can be selected which do not exist in the real Channel Impulse Response (CIR). The receiver must therefore select the true channel paths of the transmission channel among all the detected samples in a profile, in order to perform the channel estimation.
The RAKE receiver comprises a path searcher that can estimate, from a known pilot sequence, the number and locations (ie the time delays) of the paths in a frequency selective channel. These delay estimates are then used by a channel estimator to get the complex coefficients of the propagation channel and the channel impulse response over all the detected paths. Finally the combiner coherently combines the channel coefficient estimates obtained for each path to enhance the useful data signal before detection.
The invention is related to the path searcher of the RAKE receiver.
To detect paths, the path searcher uses the pilot sequence of the Dedicated Physical Channel (DPC) or any other channel. The path searcher needs a power profile prior to the path selection operation. Such a profile is preferably computed by non-coherent averaging of instantaneous channel profiles performed on a slot by slot basis. Each instantaneous profile consists in performing a series of correlation. In a CDMA system, part of the signal (called pilot sequence) is known by the receiver. At the receiver, the spread pilot sequence is generated. Correlating this signal with the received one allows estimating the energy of the paths. As the channel can be spread over a certain duration, the path searcher performs the correlation over a predetermined time window. It yields peak of energy for each path of the channel and false samples related to the noise and interference levels for other time delays. This set of values is called a profile. Then, the path searcher performs a summation of the instantaneous profiles. Such a summation allows detecting the true paths of the channel by averaging the spurious noise. The following task of the path searcher is then the thresholding of the probably spurious paths, according to predetermined rules.
From this profile, the path searcher must find the paths corresponding to the transmission channel that corresponds to the information that was originally emitted.
It is known to compute different thresholds in order to determine the good paths with low error probability. This thresholding of the detected paths is obtained by comparison of their energy with the energy of the total CIR or by the comparison with the most important path.
In general, no attempt is made to model the interference or estimate its power. A unique threshold variable is derived in order to be compared with the overall instantaneous power of the channel taps.
Usual path selection algorithms generally select a number of most powerful samples that are above a predefined fixed threshold. If the fixed threshold is too low, there is a non-negligible probability of erroneously selecting a large number of only noise samples in the profile. On the contrary, if the fixed threshold is too high, some of the channel taps with the lower power are not selected and thus a waste of energy and/or diversity is incurred.
In EP0643504 an improvement of the usual method is disclosed. A decision variable is computed and compared to a fixed quality threshold. It gives a method to compute a decision variable that is the ratio of the estimated signal power and the overall received signal consisting of the signal emitted added to the interference power. This ratio is used as a decision variable allowing the selection of the paths. In addition, in order to estimate the reliability of this decision variable, a reference threshold is computed by using the minimum Signal-To-Noise ratio (SIRmin) required by the system.
No attempt is made to explicitly estimate the interference power level. It doesn""t take into account the noise and the interference level, which is a crucial problem in CDMA systems. The estimated power is obtained by adding the energy of all the potential channel taps lying within an interval of K samples. It is therefore assumed that the exact window containing the channel impulse response is already known, but there is no method proposed to precisely track this window. Moreover, no attempt is made to distinguish the samples where effective channel taps exist from those of only noise among the K considered samples of the window. For low SIRs, the estimated threshold quality decreases rapidly; in the taps selection, it is necessary to compute a predetermined set of thresholds by simulation, which cannot be considered as an adaptive algorithm.
In xe2x80x9cMatched Filter-Based RAKE Combiner for Wideband DS-CDMA Mobile Radioxe2x80x9d, IEICE Trans. Commun. Vol.E81-B, No. 7 July 1998 (Satoru Fukumotoxe2x80x94Mamoru Sawahashixe2x80x94Fumiyuki Adachi), a threshold depending on the lowest noise level is proposed. Instead of only considering the most important sample and deriving directly a threshold, it also considers the less important sample and derives a second threshold from it. The maximum of the two threshold is used to decide if a path must be selected. This method does not take into account the whole profile and its properties. If the signal does not contain paths that are important enough, this method can consider them as noise and find a poor quality signal.
The invention gives a new method for finding the paths corresponding to the transmission channels, which determine dynamically a reliable adaptive threshold based on the noise and the interference level.
The goal of the invention is to select reliably the channel taps using a large power profile (large time observation window or large number of samples) with possible non-coherent averaging in time.
The invention consists in a path selection method in a CDMA receiver for the selection of the true paths of a transmission channel, comprising the step of comparing the transmission channel energy with a threshold. The threshold is dynamically determined as a function of the mean of noise and interference samples derived from the transmission channel profile.
The noise and interference paths can be derived from the transmission channel profile by suppressing at least one of the most important estimated paths. These most important paths will represent about the tenth of the samples.